The electrostatic spraying of powders, and the electrostatic fluidized bed deposition of powders, has been known to the art. The process of electrostatically spraying powders generally involves establishing an electrical field, within a spray gun or other apparatus, which is capable of charging the powder particles. The charge on the particle directs and causes the particle to deposit on the desired object, and in many cases a complete uniform coating is obtained over the entire object, even though the electrostatic spray gun is directed towards only one face thereof.
The electrical charge given to a particle during electrostatic coating may be represented by the following general formula: EQU q= kE.sub.z a.sup.2
wherein k is a factor which depends upon the nature and the shape of the particle, E.sub.z is the electric field in the charging zone, and a is the average radius of the particle.
The electric charge thus is dependent upon the field intensity (E.sub.z) and on the surface area (and therefore the radius) of the particle. The smaller the particle size, the higher the electrical charge in relation to the particle mass (and the mass is proportional to a.sup.3). Each charged particle during the electrostatic coating, e.g., spraying, operation is subjected to an electrostatic force F= qE, with E being the electric field existing around such particle at a given moment.
With the electrostatic spraying technique, the powder is charged and adheres to a heated or an unheated substrate for a period generally sufficient to permit conveying the coated object to an oven. A subsequent bake, or curing, process in the oven transforms the powder into a smooth, uniform coating having desired characteristics. Some of the main advantages of the electrostatic spraying process are the fact that no solvents are used, and therefore no solvent costs are involved and the coating operation is much safer. Generally, any excess powder can be recovered from the spray booth and reused, which, together with the fact that very little overspray is encountered, results in almost negligible powder loss. In many situations, a coating of appreciable thickness can be built up in a single operation, as compared to the need in conventional paint operations, to use several coatings to produce the same thickness.
The prior art has been unable to apply, through electrostatic means, a layer of a conductive material, e.g. a conductive metal such as zinc, as the conductivity of the powder results in a shorting of the electrostatic apparatus. Therefore, the prior art, when wishing to apply superimposed layers of various materials including at least one conductive material, has applied such layers separately, with a non-electrostatic application means being used to apply the conductive material.
Besides being unable to apply layers of conductive materials, the prior electrostatic methods of coating have suffered another distinct disadvantage in the application of a plurality of coatings to a substrate, with baking between the coating steps. Such plural coating operations have generally produced a resulting coating having a plurality of layers with such poor adhesion between the different layers that delamination may occur.
The prior art has utilized mixtures of polymeric materials, e.g., thermosetting polymers, and certain powdered metals for decorative effects. For instance, furniture manufacturers frequently electrostatically spray a mixture of powdered epoxy resin and powdered, flaked aluminum or bronze, the powder mixture containing about 2% by weight of metal, on furniture. In both cases, the metal migrates during the baking operation to the surface of the coating, providing an attractive metallic finish.
French Pat. No. 1,261,473 relates to the electrostatic spraying of a polymer such as a cellulose ester. The patent discloses that powdered aluminum may be added to the plastic powder to improve the chargeability thereof. However, the French patent makes no mention of the amount of aluminum powder added to the cellulose ester or polyethylene powder, and it is clear that the patentee must be contemplating relatively small amounts of powdered aluminum, as very small amounts of the finely powdered aluminum should be sufficient to change the chargeability of the polymeric powder. In addition, even relatively low amounts of powdered aluminum, e.g. 1/2 or 1% by weight of the total composition, would result in migration of aluminum to the surface of cellulose ester coatings during the bake cycle. To avoid this metallic top coat, it is believed clear that the French patent must be concerned with very small amounts of powdered aluminum.